1. Field of the Invention
The present invention relates to continuous ink jet printing apparatus and more particularly to improved systems (i.e. structural and functional modes) for self-cleaning of the print head assembly of such apparatus.
2. Description of the Prior Art
The term "continuous" has been used in the field of ink jet printer apparatus to characterize the types of ink jet printers that utilize continuous streams of ink droplets, e.g. in distinction to the "drop on demand" types. Continuous ink jet printers can be of the binary type (having "catch" and "print" trajectories for droplets of the continuous streams) and of the multi-deflection type (having a plurality of print trajectories for droplets of the continuous streams). Binary type apparatus most often employs a plurality of droplet streams while multi-deflection apparatus most often employs a single droplet stream.
In general, continuous ink jet printing apparatus have an ink cavity to which ink is supplied under pressure so as to issue in a stream from an orifice plate that is in liquid communication with the cavity. Periodic perturbations are imposed on the liquid stream (e.g. vibrations by an electromechanical transducer) to cause the stream to break up into uniformly sized and shaped droplets. A charge plate is located proximate the stream break-off point to impart electrical charge in accord with a print information signal and charged.droplets are deflected from their nominal trajectory. In one common binary printing apparatus charged droplets are deflected into a catcher assembly and non-charged droplets proceed along their nominal trajectory to the print medium.
The components described above (particularly the orifice plate and charge plate) must be precisely sized and positioned to achieve accurate droplet placement on the print medium. However, even after such careful manufacture, significant problems are often presented when the apparatus is shut down for extended periods (e.g. overnight). That is, ink residue which remains from previous usage will often dry in the print head during such shut-down periods. If the dried residue is in the orifice plate it can cause crooked jets. If dried ink residue is in the print head cavity it can become dislodged during printing operation and cause blockage of an orifice or a crooked jet. Dried ink residue in other parts of the circulation system can be filtered; however, excessive quantities of such residue necessitates frequent filter maintenance.
Prior art solutions to the residue problems have included (i) purging the ink cavity, orifice plate and charge plates with air upon shut-down of an operational cycle; (ii) providing a nearly instantaneous negative pressure at shut-down to avoid the residue on the lower print head and (iii) introduction of cleaning solution at start-up and or shut-down.
These solutions are all helpful but not without related difficulties or disadvantages. For example, purging the ink system with air and/or a cleaning solution adds considerable complexity to the apparatus as well as necessitating a lengthy flushing period at start-up. The instant shut-down approach requires an extremely fast-actuation solenoid valve and is not completely reliable in constructions where jet-to-electrode clearances are very small.
U.S. application Ser. No. 06/722,551, entitled "Ink Jet Printing Apparatus Having a Wet-Storage System", and filed Apr. 12, 1985, in the name of M. Piatt, discloses a highly useful approach for solving the above-noted problems. This approach provides a unique home station into which the apparatus print head assembly is transported from the operative printing path for wet storage and start-up procedures. The present invention provides further improvements in the approach described in the aforementioned copending application and in particular provides structure and operational modes which effect enhanced cleaning of the ink jet orifice(s) and the cleaning of other operative portions of the lower print head assembly.
The present invention also constitutes an improvement upon the cleaning system disclosed in commonly assigned U.S. application Ser. No. 495,183, entitled "Fluid Jet Printer and Method of Ultrasonic Cleaning", and filed May 16, 1983, now U.S. Pat. No. 4,563,688 in the name of Hilarion Braun, which discloses an ink jet printing apparatus wherein the orifice plate is "self-cleaned" by the imposition of predetermined ultrasonic vibrations.
SUMMARY OF THE INVENTION
Thus, one general objective of the present invention is to provide improved ultrasonic cleaning for operative portions of ink jet printing apparatus. A more particular purpose is to provide structural configurations and functional techniques that facilitate the ultrasonic cleaning of portions of the charge plate and/or catcher structure of the print head assembly as well as enhanced cleaning of the orifice plate structure of such apparatus. The present invention, when employed separately or in combination with other self-cleaning features disclosed in the above-noted applications, provides significant advantages by obviating more complicated and time consuming cleaning approaches and by providing enhanced apparatus performance.
The above and other objects and advantages are achieved in accord with the present invention by providing for ink jet printing apparatus of the type having: a print head body including an ink cavity, an orifice plate in liquid communication with the ink cavity, and a charge plate located in a spaced relation to the orifice plate, the improvement comprising: means for supporting an ink mass against gravitational forces, in contact with both the orifices of the orifice plate and the drop charging surface of the charge plate and means for imparting ultrasonic cleaning vibrations to such a supported ink mass.
In one preferred embodiment such ink mass support is provided by capillary forces between the charge plate and an opposing wall member and the ultrasonic vibrations are provided by a stimulating transducer on the print head body and transmitted to the charge plate surface by the supported liquid.
In another aspect the present invention provides ink jet printer apparatus having an improved orifice-cleaning function and structure wherein ultrasonic cleaning vibrations are applied while ink is cross-flowing through the print head. In a preferred embodiment in accord with this aspect, such cleaning vibrations are applied concurrently with a variation of the pressure differential across the orifice plate to effect oscillation of ink into and out of the ink orifices.